Salisbury's FTTH Blog

A running update of Salisbury's FTTH project

Splitter Update

I had someone point out to me that I missed one important aspect of our network design in my post from this afternoon. I explained how the city had gone from using 32 way splitters to 16 way and how that change was going to allow the city to provide even higher bandwidth to our subscribers. But if you had paid really close attention, you would have noticed one interesting aspect of that discussion/

Earlier I explained what an OLT was and how one port, or connection, fed one splitter. This splitter could feed up to 32 homes/businesses if it was a 32 way splitter or 16 homes if it was a 16 way splitter. What maybe wasn’t quite as obvious is that this could lead to a much greater cost to the city.

Say we have 320 customers we want to sign up. If we use 32 way splitters, then we would need to use 10 ports on the OLT (again, each port can feed 32 homes/businesses). But if we use 16 way splitters, then we would need 20 ports. We haven’t bid out the GPON equipment yet so I can’t say what each port costs, but they are not inexpensive. So it would look like the 16 way splitters will almost double our costs. But what we are going to do is use 2 way splitters in the headend to feed two 16 way splitters in the field. This means we will effectively still be using 32 way splitters (one port on the OLT will feed a 2 way splitter that will then feed two 16 way splitters) in the beginning thus saving money. But once the number of subscribers grows or the demand for bandwidth increases, then we will be able to remove the 2way splitters. By this point we will have enough subscribers, and will be making enough money, to afford the costs of the extra OLT ports. This provides something similar to a “pay as you go (or grow)” plan.

I know this is more technical than most of you want to know, and I’ve tried to make it as easy to understand as I could. What I’m trying to show is that a lot of thought has gone into the design of the network and we’re trying to do this as inexpensively as we can. The network as it’s being built, can grow with the city and we can spread the cost over several years. Yet when we’re five or ten years into the project, the city will still have a state of the art network that will have the bandwidth the users want.

If you want to discuss this further, and I can’t believe anyone would, then let me know and I’ll do my best to confuse you even further. I’ll try not to make my next posts quite as boring.

June 18, 2009 Posted by Michael Crowell | Uncategorized | | No Comments Yet

Tuesday’s Council Meeting Updated

One other item that I mentioned at Tuesday’s council meeting that I forgot to mention in my last post, was the discussion about Salisbury using 1×16 splitters in place of 1×32. I didn’t really forget it, it was just late and my mind was too fuzzy to try and explain it. So I thought I would try today.

Salisbury is using a technology called GPON (Gigabit Passive Optical Network) to provide service. In a GPON system, a strand of fiber leaves the headend and runs to a LCP (Local Convergent Point) cabinet. In the cabinet is a device called a splitter. This is a passive device (doesn’t require electricity) that takes the lightwave signal it receives and splits it. The GPON standard allows for up to 64 way splitters. We were originally going to use 32 way splitters. At the urging of AEG, we have now decided to use 16 way splitters.

I know this doesn’t mean a lot right now but let me see if I can explain it. A GPON system has two major pieces of equipment, the OLT (Optical Line Terminal) and ONT (Optical Network Terminal). The OLT is located in the headend and the ONT at the homes/businesses. The OLT is the device that takes signals from all the electronics that generate the video, phone, Internet and any other service we may provide, and turns it into light pulses that travel down the fiber. Each port (a connection that feeds one fiber) can provide 2.5G of bandwidth downstream and 1.2G upstream. Each port feeds one splitter so if we were using 32 way splitters, then each home could receive 75Meg down, 37.5Meg up. Since we try and talk about symmetric bandwidth (the upstream and downstream bandwidth being the same) and we try to round down to a number ending with a zero, we usually say we can provide up to a 30Meg symmetric connection using 32 way splitters. You can see we can offer a much greater speed than we are actually saying. We just don’t want to get into the same habit as the current providers by saying we can provide high speeds when we really can’t.

Now if we use 16 way splitters in place of 32 way, then the numbers change rather dramatically. If you now take the 2.5G available downstream bandwidth from one port and divide by 16, you get 150Meg down and 75Meg up. So with 16 way splitters we can offer a 70Meg symmetric connection.  This means we can provide enough bandwidth for now and in the foreseeable future. This doesn’t mean you have to sign up for this speed. We are going to offer in our base package a 10Meg symmetric service. If you want more speed, there will be options for additional bandwidth. As a side note we will also have ONT’s that can deliver 1Gigabit Per Second to businesses/schools and any high demand customers.

Just like with computers, the GPON system is continually being improved. Industry is already working on equipment that can provide 10G speeds so at some point, all we will have to do is replace the OLT and ONT’s and we will be able to provide even higher speeds. The good thing is, the OLT and ONT’s don’t have to be all replaced at the same time, we can do it over several years as the demand occurs. This is the reason we say FTTH, especially the fiber, is future proof.  

This is already too long, so I will wait until another day to discus what the ONT does and how it works. I know you can hardly wait.

June 18, 2009 Posted by Michael Crowell | Technical | | No Comments Yet